Fluid flow sensing device

ABSTRACT

The disclosed device includes a thermistor sensor particularly adapted for disposition in thermal communication with a fluid medium utilized for effecting cooling of equipment. The thermistor sensor senses variations in the heat transfer characteristics of the medium, and is coupled to a power supply through a resistor, which cooperates with the thermistor sensor to define a voltage divider, including a junction. A voltage responsive means is coupled to the junction and is rendered operable in response to the establishment of an increased voltage level at the junction, when the heat transfer properties of the medium decreases. The voltage responsive means provides an electrical signal for energizing a switch means so as to render a load circuit inoperable in response to a decrease in the heat exchange properties of the fluid medium. Alternatively, the electrical signal may be utilized for operating an indicator to provide an indication of a decrease in the heat exchange properties of the fluid medium.

[ Dec. 25, 1973 FLUID FLOW SENSING DEVICE [75] Inventor: Clifford A.Budge, Attleboro, Mass.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

[22] Filed: Jan. 7, 1972 21 App1.No. :216,299

Related US. Application Data [63] Continuation of Ser. No. 811,869,April 1, 1969,

323/68, 69; 317/1485 B, DIG. l, DIG. 3, 132; 73/204, 295; 307/118, 310,318

References Cited UNITED STATES PATENTS 2,769,121 10/1956 Rogoff 317/1323,032,690 5/1962 Elliot 317/41 3,184,689 5/1965 Wylde 331/117 3,307,1672/1967 Race 340/253 3,407,840 10/1968 Finnegan 137/392 3,432,840 3/1969Neapolitakis et a1. 340/244 R 3,498,131 3/1970 Rickey 340/244 C FOREIGNPATENTS OR APPLICATIONS 1,178,616 9/1964 Germ any Primary Examiner-lohnW. Caldwell Assistant Examiner-Daniel Myer Att0rneyHarold Levine [57]ABSTRACT The disclosed device includes a thermistor sensor particularlyadapted for disposition in thermal communication with a fluid mediumutilized for effecting cooling of equipment. The thermistor sensorsenses variations in the heat transfer characteristics of the medium,and is coupled to a power supply through a resistor, which cooperateswith the thermistor sensor to define a voltage divider, including ajunction. A voltage responsive means is coupled to the junction and isrendered operable in response to the establishment of an increasedvoltage level at the junction, when the heat transfer properties of themedium decreases. The voltage responsive means provides an electricalsignal for energizing a switch means so as to render a load circuitinoperable in response to a decrease in the heat exchange properties ofthe fluid medium. Alternatively, the electrical signal may be utilizedfor operating an indicator to provide an indication of a decrease in theheat exchange properties of the fluid medium.

10 Claims, 8 Drawing Figures PATENTED IJEEZ 5 I973 SHEET 1 BF 2 vINVENTOR, CZzfford A. Badg BY 9%; 7/ W A ttfy.

FLUID FLOW SENSENG DEVKCIE This is a continuation, of application Ser.No. 81 1,869, tiled Apr. 1, 1969, and now abandoned.

The present invention relates generally to a device for sensingvariations in the heat exchange properties of a medium and moreparticularly is directed to a device for sensing variations in the heatexchange properties of a fluid medium as an indication of the relativeflow rate of the medium.

It has become increasingly important in recent years to provide fluidcooling systems for use in conjunction with heat sensitive equipment.Such cooling systems are especially desirable in conjunction withequipment employing various types of electrical circuit elements,particularly semiconductor devices, which are generally relativelyimmune to mechanical shock, but may be highly sensitive to increases inambient temperature. in addition, the use of equipment includingrelatively large numbers of such circuit elements or other heatsensitive components provided in an enclosed space, such as incomputers, for example has become increasingly prevalent.

it has been found that in many instances appropriate cooling may beeffected by merely establishing a suitable air flow and utilizing theheat transfer characteristics of this environmental air in order toeffect appropriate cooling of the circuit elements. Alternatively, inother instances it may be desirable to dispose the equipment incommunication with another gaseous medium or in an appropriate liquidmedium. In any event the medium may be arranged to have the desired heattransfer characteristics by the provision of means for maintaining thetemperature of the medium at a desired level or by selecting a medium,which has the desired heat transfer characteristics, when a preselectedflow rate or velocity is established relative to the equipment beingcooled.

However, in order to provide assurance of the continued exposure of theequipment to the cooled or flowing medium it is desirable to provide adevice for sensing the presence of an appropriate flow velocity ormedium temperature. It may be readily appreciated that if such 7 acooling system should fail, even for a relatively short time interval,potentially destructive heating effects may result, causing irreparabledamage to various of the heat sensitive components of the equipment.Consequently, the necessity for the provision of a device becomesreadily apparent, for continually monitoring the temperature, presence,or flow rate of the medium and providing an appropriate indication ofthe failure of the cooling system, or, alternatively, effectingautomatic shut-down of the equipment being cooled.

Although various types of electro-mechanical devices have been developedfor sensing the cooling effects of fluid flow such sensors are ofteninadequate for various reasons. For example, such sensors may have poorresponse characteristics, and may be unable to sense relatively smallchanges in fluid flow. In addition, such sensors, particularly whenrelatively large in size are susceptible to nuisance tripping as aresult of temporary surges in fluid flow, equipment vibration, equipmentorientation, gravity, and mechanical stress or vibration. Furthermore,the provision of electromechanical sensors presents a reliabilityproblem due to eventual wear of mechanical parts.

Accordingly, it is an object of the present invention to provide animproved device for sensing the heat transfer characteristics of a fluidmedium.

It is another object of the present invention to provide an improvedprotective device for sensing variations in the flow velocity of a fluidmedium as an indication of the heat transfer characteristics of themedium.

it is another object of the present invention to provide a substantiallyfail-safe protective device for use with equipment to be arranged incommunication with a cooling fluid medium for sensing variations in theheat exchange properties of the medium as an indication of variations inthe relative velocity of the medium.

It is still another object of the present invention to provide aprotective device adapted for sensing variations in the heat transfercharacteristics of a fluid coolant to permit relatively rapid disruptionof the power supplied to the equipment being cooled.

It is a further object of the present invention to provide a protectivedevice adapted for sensing variations in the heat transfercharacteristics of a fluid coolant and for effecting energization of anindicator in response to a decrease in the heat transfer characteristicsof the fluid.

It is still a further object of the present invention to provide aprotective thermal sensing device adapted for sensing variations in theheat transfer characteristics of a fluid coolant as an indication ofvariations in the flow rate of the fluid which protective thermalsensing device is extremely flexible and versatile in operation anddurable in use.

Various additional objects and advantages will become readily apparentfrom the following detailed description and accompanying drawingswherein:

FIG. 1 is a graphical representation of a typical resistance temperaturecurve for a positive temperature coefficient thermistor;

FIG. 2 is a graphical representation of typical voltage-current responsecurves of a positive temperature co-efficient thermistor sensorsubjected to several different heat transfer conditions and alsoillustrating a superimposed series resistance load line; and

FIGS. 3-8 are schematic circuit diagrams of various embodiments ofadevice in accordance with the principles of the present invention.

Corresponding elements are provided with corresponding referencenumerals in the drawings.

Referring generally to the drawings, a schematic circuit diagram of oneembodiment of a device in accordance with the principles of the presentinvention is illustrated in H0. 3. As shown, a thermistor sensor 10 isprovided which is adapted to be disposed in thermal communication with asuitable fluid medium. The fluid medium may comprise a gaseous medium,such as air, or it may comprise a desired liquid medium. Similarly, thethermistor sensor may be arranged to sense the flow rate of the fluid,the temperature of the fluid, the presence or absence of the fluid,etc., depending upon the contemplated application of the device. Thethermistor sensor 10 is coupled to a suitable power supply (not shown)through a calibration means 12, preferably comprising a resistorconnected in series relationship with the thermistor sensor. Thethermistor sensor 10 and the series resistor 12 are arranged in avoltage divider configuration, which includes a junction 14 defined bythe connection between the thermistor sensor and the resistor. Thevoltage divider junction 14 is cou- 'pled to a suitable voltageresponsive means 16, preferably through a current limiting resistor 17.The voltage responsive means 16 is adapted to sense variations in thevoltage levelestablished at junction 14, as a result of variations inthe resistance of thermistor 10, to provide an electrical signal inresponse to a preselected increase in this voltage level, as will bemore fully explained hereinafter. In addition, a suitable power sensingmeans 18 is preferably connected to the'voltage responsive means. Thepower sensing means 18 is adapted to be energized in response toenergization of the voltage responsive means 16, and may be arranged toprovide an appropriate indication, when the voltage responsive means isenergized. Alternatively, the power sensing means 18 may comprise anappropriate switch such as a relay coil, which is coupled to theequipment being protected for selectively disrupting the power beingsupplied to the equipment in the absence of desired heat transferconditions.

Referring briefly to the graphical representations in FIGS. 1 and 2,operation ofa device in accordance with the present invention isexplained in somewhat more detail. More particularly, FIG. 1 illustratesthe resistance-temperature characteristic curve 19 of a typical positivetemperature co-efflcient thermistorsensor.'As may be seen from aninspection of FIG. 1, the resistance of such a thermistor sensor remainsrelatively constant with increasing temperature until a temperaturedesignated T, is reached. T represents the anomaly temperature of thethermistor sensor, and designates the temperature at which theresistance of the thermistor sensor changes by several orders ofmagnitude with very small increases in temperature. This particularproperty of a positive temperature co-efficient thermistor sensor isadvantageously utilized in the present invention.

For example, in operation of a device in accordance with theprinciplesof the present invention a relatively.

small energizing voltage supplied to the thermistor sensor causesself-heating to be effected. However, as long as the thermistor sensoris exposed to a cooling me dium, such as an air flow, it may be cooledsufficiently to cause its resistance to remain in the stable regionbelow the anomaly point. Thus, it may be seen that a reduction in theheat transfer characteristics of a medium may be readily sensed by anappropriately arranged thermistor sensor.

In this regard, referring to FIG. 2, which depicts the voltage-currentcharacteristics of a typical positive temperature co-efficientthermistor exposed to a fluid medium at several different heat transferconditions with a superimposed series resistor load line, operation of adevice in accordance with the principles of the present invention isfurther illustrated. More particularly, a curve 20 is shown illustratingthe voltagecurrent characteristic of such a thermistor sensor disposedin a fluid medium, such as air having a desired flow velocity sufficientto cool the thermistor sensor, preventing it from reaching its anomalytemperature, and sufficient to maintain the equipment being cooled (notshown) within the requisite temperature limits. Similarly, the curvedesignated by the numeral 22 represents the voltage-currentcharacteristic curve of the same thermistor sensor,.but exposed to thefluid medium under conditions of a substantially decreased flow velocitywhich is insufficient for cooling the thermistor sensor to preventself-heating and which could be insufficient for cooling the equipment.As illustrated, a significant shift in the relative position of thevoltagecurrent characteristic curve results under conditions ofdecreased fluid velocity and associated decreased heat transfer.

. As previously explained, a series resistor 12 is generally coupled tothe thermistor sensor, and the load line of the resistor 12 isillustrated in FIG. 2 and designated by the numeral 24. In addition, itmay be noted that the relative position of the resistance load line 24is dependent upon the resistance of series resistor 12, and may bereadily shifted in position in accordance with the circuit requirementsby selecting a series resistor having a different value of resistance.As shown, the resistance load line 24 intersects the fluid velocitycurve 20 at a particular operating point designated I V representing theoperating point of the thermistor sensor 10 during usual operation ofthe circuit, when the thermistor sensor is exposed to a medium having asufficient velocity flow rate, or heat transfer condition, necessary foreffecting the requisite cooling of the equipment. If desired, theresistance load line 24 may be shifted in position to provide adifferent operating point, I V as previously explained. Furthermore,observation of FIG. 2 reveals that the resistance load line 24 alsointersects the curve 22, which represents the voltagecurrentcharacteristic of the thermistor sensor at a decreased fluid velocity.Since a positive temperature coefficient thermistor is utilized, theresistance of the thermistor increases, when it is exposed to anincreased temperature or decreased heat transfer conditions, as thethermistor approaches and reaches its anomaly point. Thus, the voltageacross the thermistor increases, while the current flow decreases. Thiseffect is illustrated by the intersection of the resistance load line 24with the curve 22. As shown, a substantially larger voltage and asubstantially smaller current occur. The intersection of the resistanceload line 24 with the curve 22 represents the operating point, V 1 ofthe thermistor sensor 10, when exposed to decreased heat transferconditions. More particularly, V represents the-voltage appearing at thejunction 14 under such conditions, while represents the current throughthe thermistor.

Thus, during operation of a device in accordance with the presentinvention the energizing voltage, which is supplied to the thermistorsensor 10 causes some self-heating of the thermistor sensor. However, aslong as the thermistor sensor is disposed in a cooling medium, such asan adequate air flow, the sensor is sufficiently cooled to remain stablealong its low resistance region. However, when the air flow velocitydecreases to a certain level so that the equipment being cooled, forexample, may be in danger of overheating, this decreased heat transfercondition disturbs the stability of the thermistor sensor and itstemperature begins to increase. As the temperature of the thermistorsensor increases and approaches its anomaly point, its resistanceabruptly increases by several orders of magnitude. This increase inresistance is accompanied by a rapid voltage rise at the junction 14 towhich the thermistor is coupled, and is also accompanied by a rapiddecrease in the current through the thermistor. As a result of thisrapid voltage rise at the junction 14 and the reduced current throughthe thermistor, current is shunted away from the thermistor, and theincreased voltage level is effective to trigger the voltage responsivemeans 16, rendering it conductive. This switching action may be furtherutilized for activating a warning device, such asan indicator light,alarm buzzer, etc., or, alternatively, may be utilized for operating acircuit breaker or the like in order to disrupt the power being suppliedto the equipment, which is exposed to insufficient cooling. in addition,since current is shunted away from the thermistor as its temperatureincreases, the inherent fail-safe operation of the device may be noted,whereby burn-out of the thermistor is prevented because increasedtemperature results in decreased current flow through the thermistor.

Referring now to FIG. 4 one preferred embodiment of a device inaccordance with the principles of the present invention is illustrated.As shown, the thermistor sensor is coupled to a source of power (notshown) through the series calibration resistor 12, and defines a voltagedivider therewith, having a junction 14. The junction M is coupled to agate electrode 26 of a silicon controlled rectifier 28 through asuitable coupling means 30, such as a current limiting resistor.lnaddition, a transient suppression device, such as a capacitor 32, iscoupled between the gate electrode 26 and ground in order to preventinadvertent triggering of the silicon controlled rectifier as a resultof transient electrical signals. Preferably, a suitable current sensingmeans 33 is coupled to the anode cathode circuit of the siliconcontrolled rectifier 28 and is adapted to be energized in response toconduction of the silicon controlled rectifier. The means 33 maycomprise a suitable indicator device such as, an indicator light, analarm buzzer, or the like, which is rendered operative in response toconduction of the silicon controlled rectifier 28, in .order toprovidean indication that the stability of the thermistor has been disturbedand that insufficient cooling is being provided. Alternatively, themeans 33 may comprise a relay coil, for example, which is operativelycoupled to a pair of relay contacts coupled to a load circuit, which maybe connected to the equipment being cooled, so that energization of therelay coil may effect opening of normally closed relay contacts, therebyeffecting turn-off of the equipment in response to a decreased heattransfer condition of the medium.

Referring to FIG. 5 another embodiment of the present invention isillustrated. In this embodiment the thermistor It) is similarly coupledto a source of power (not shown) through a series resistor 12 anddefines a voltage divider configuration therewith, including thejunction 14}. ln operation, as previously explained, as the heattransfer characteristics of the medium in which the thermistor sensor isdisposed decreases and the resistance and temperature of the thermistorincrease, the voltage at the junction 14 increases to a preselectedlevel, for example, the voltage level V (referring to FIG. 2), while thecurrent through the thermistor decreases to the level, [1,, as theanomaly point is approached. in the illustrated embodiment the junctionM is coupled to a voltage responsive device 34, preferably comprising avoltage regulator diode, having a threshold voltage, which issubstantially equal to or slightly less than preselected voltage level VAs illustrated, the voltage regulator diode 34 is reverse biased by thepositive voltage at the junction 14, and may be rendered conductive inresponse to the establishment of the preselected voltage level at thejunction 14. A suitable current limiting resistor 36 is also providedfor connecting the cathode of the voltage regulator diode to the voltagedivider junction l4, while the anode of the voltage regulator diodeconnected to the base electrode 38 of a switching transistor 40. Theswitching transistor is rendered conductive in response to energizationof the voltage regulator diode 34 and in the illustrated embodiment isconnected to a selectively energizable relay coil 42 operativelyconnected to a pair of normally closed relay contacts 44, which areadapted to selectively energize or de-energize an external load circuitin response to energization of the relay coil.

In this regard, it may be noted that the use of the terms energizationor ole-energization in referring to effecting operation or temporarydisruption of an external load circuit are used interchangeably hereinto denote an effect on the operation of the electrical circuit supplyingpower to the equipment being cooled. For example,'in certain instancesthe relay contacts may be coupled to a circuit breaker which is renderedoperative to de-activate an external load circuit in response to closingof normally open relay contacts. Similarly, the relay contacts may bemaintained normally closed, as illustrated in FIG. 5, and coupleddirectly to an external load circuit in a manner so that opening of therelay contacts de-activates or interrupts the load circuit, wheninsufficient cooling occurs.

Referring to FIG. 6 an alternative embodiment of the present inventionis illustrated including a signal light indicator, which is normally onduring operation of the circuit, to indicate that the requisite coolinglevel is being sensed by the thermistor sensor, thereby indicating thatthe equipment is being exposed to sufficient cooling. In addition, whenthe thermistor sensor detects a decrease in the heat exchange propertiesof the fluid medium in which it is disposed, power is shunted away fromthe indicator, and another indicator such as a light of a differentcolor or an alarm buzzer is energized to indicate the presence ofinsufficient cooling. Alternatively, if desired, a suitable relay coiland associated relay contacts may be provided coupled to an externalload circuit for effecting a circuit breaking function in response tothe presence of insufficient cooling.

More particularly, referring in detail to FIG. 6 the thermistor sensor10 is connected to a power supply (not shown) through the seriesresistor 12 to form the voltage divider configuration, including thejunction 14. In addition, a first switch means 46 preferably comprisinga transistor is coupled to the power supply through a current limitingresistor 48, as shown. A suitable indicator 50, such as an indicatinglight, is provided connected between the power supply and the firstswitch transistor 46, arranged such that the indicator light 50 isenergized or lit, while the switch transistor 46 is in a conductivestate due to the application of a signal to its base electrode throughthe resistor 48. A second switching means 52, preferably comprising asecond switch transistor, which is normally nonconductive is coupled tothe voltage dividerjunction 14 through a voltage responsive means 54,such as a voltage regulator diode. The voltage regulator diode 54 haspredetermined threshold voltage, which is in excess of the usualoperating voltage developed at the junction 14, when the thermistorsensor 10 is being sufficiently cooled. In this connection, the voltageregulator diode preferably has a threshold voltage substantially equalto or somewhat less than the voltage level V (referring to 7 F IG'. 2),i.e., the voltage level developed at the junction 14, when thethermistor sensor is not sufficiently cooled and changes to a highresistance condition.

The--transistor52 is also coupled to another indicating means 56, whichis adapted to be energized in response to conduction of the transistor52. In this re gard, the transistor 52 is normally non-conductive, aspreviously mentioned, until the threshold voltage of the voltageregulator diode is reached which renders the transistor 52 conductive,causing electrical power to be shunted away from the base of transistor46. Thus, when the transistor 52 is rendered conductive and electricalpower is removed from the base of transistor 46, rendering transistor 46non-conductive, energizing power is also removed from indicator light50. Similarly, when transistor 52 is rendered conductive, it energizesthe indicator 56. In those instances, where the indicator 56 comprisesan indicator light, such indicator light is preferably of a differentcolor from the indicator light 50, so as to provide a visual indicationthat the thermistor sensor 10 is no longer being exposed topropercoolant flow. Alternatively, indicator 56 may comprise an alarmbuzzer, or the like, to provide an audio indication of reduced heattransfer characteristics. If desired, the indicating means 56 may alsoinclude a "suitable relay coupled to an external circuit breaker, aspreviously explained, so as to provide an indication ofimproperoperation, as well as de-energizing the external load circuit.

' Referring to FIG. 7, another alternative embodiment of a device inaccordance with the principles of the present invention is illustrated.The illustrated embodiment is particularly adapted for effectingmulti-point sensing of theheat exchange properties of a fluid medium. Asillustrated, a plurality of thermistor sensors, designated by thenumerals 19a, 10b, and 10c, are provided. These thermistor-sensors arepreferably arranged in mutually spaced relationship with respect to eachother in the fluid medium. For example, the thermistor sensors may bedisposed in different locations for sensing the air flow being suppliedto different regions of the equipment which is being cooled and forenergizing a suitable alarm, circuit breaker, etc., when the coolantflow at any of these locations is reduced or disrupted.

More particularly, referring in detail to the embodiment, illustrated inFIG. 7, each of the thermistor sensors- 10a, 10c is coupled to a powersupply (not shown) through an associated series resistor 12a, 12b, and120 respectively to def ne a voltage divider config uration. Inaddition, each of the respective voltage divider, arrangements includesa junction 14 a, 14 b, and 14 c. The respective junctions 14 a 14 c areelectrically connected in parallel relationship to a voltage responsivemeans 58 preferably comprising a voltage regulator diode, having apreselected threshold voltage, so that a rise in the resistance of anyof the thermistor sensors 10 a 10 c and an associated rise in thevoltage at one of the respective junctions 14 a l4 cin excess of thethreshold voltage is effective to energize the voltage regulator diode.In this connection the voltage regulator diode 58 is also coupled to asuitable switch means 60 preferably comprising a silicon controlledrectifier, having agate electrode 62 connected to the voltage regulatordiode, so that triggering of the silicon controlled rectifier 60 resultsin response to energization of the voltage regulator diode 58.Accordingly, when the threshold voltage of the voltage regulator diode58 is exceeded, a signal is applied to the gate electrode 62 of siliconcontrolled rectifier 60, to trigger the silicon controlled rectifierinto conduction. I

In addition, a suitable indicating means 64 is preferably provided inthe anode-cathodecircuit of thesilicon controlled rectifier 60, and isadapted to be energized in response to triggering of the siliconcontrolled rectifier. The indicating means 64 may comprise a suitableindicator light, an alarm buzzer, etc., for providing an indication thatinsufficient cooling is sensed by one of the thermistor sensors 10 a 10c. If desired, the indicating means 64 may include a suitable relay coilcoupled to an associated pair of relay contacts in an external loadcircuit, arranged such that energization of the relay coil effects acircuit breaking function, when one or more of the associated thermistorsensors 10 a 10 c senses the presence of insufficient coolant flow.

As an additional feature, in the embodiment illustrated in FIG. 7, thevoltage regulator diode 58 is arranged to have a preselected threshold,voltage level, which is sufficient to prevent undesired triggering ofthe silicon controlled rectifier during usual operation. In this regard,a relatively small electrical signal maybe applied to the voltageregulator diode 58 from each of the associated junctions 14 a 14 c, evenin the presence of the requisite coolant flow, and in certain instancesthe summation of these signals may approach the gating level of thesilicon controlled rectifier. Accordingly, the voltage regulator diode58 is selected such that its threshold voltage is in excess of thesummation of such signals in order to prevent inadvertent triggering ofthe silicon controlled rectifier during usual operation of the system,until one or more of the thermistor sensors is insufficiently cooledsuch that its resistance increases substantially and a voltage level isestablished at one of the junctions 14 a 14 c, approaching the voltage V(referring to FIG. 2).

It is also generally desirable to provide isolation means 66 a, 66 b,and 66 c between each of the respective junctions 14 a 14 c'and thevoltage regulator diode 58. The isolation means 66 a 66 c preferablycomprises a plurality of steering diodes, with, one of such steeringdiodes being connected between each of the respective junctions 14 a 14c and the voltage regulator diode. In operation, the steering diodesfunction to assure the maintenance of electrical isolation between therespective thermistor sensors, when a particular thermistor sensor issubjected to decreased cooling signal is energized sufficiently toeffect establishment of a voltage level in excess of the voltageregulator diode threshold voltage at its associated thermistorresistorjunction, thereby preventing sympathetic operation of the otherthermistor sensors. Thus, it may be seen that operation of theembodiment illustrated in FIG. 7 is substantially similar to that of thepreviously described embodiments. When any one or more of the thermistorsensors 10 a 10 c is exposed to a decreased heat transfer condition ofthe fluid medium, the resistance of the particular thermistor sensorincreases as previously explained, which results in the establishment ofa substantially increased voltage level at its associatedthermistor-resistor junction effecting energization of the voltageregulator diode 58 and consequent triggering of the silicon controlledrectifier 60, which in turn operates the indicating means 64.

Referring to FIG. 3 an alternative embodiment of the present inventionis illustrated which includes a thermistor sensor arranged substantiallyindependent of minor changes in ambient temperature, while maintain-.ing its sensitivity to variations in the heat exchange properties of afluid medium, such as represented by variations in the flow rate. Moreparticularly, the thermistor sensor M) is coupled to a power supply (notshown) through the serially connected resistor 12, to form the voltagedivider configuration having the junction 14, as previously explained.However, the thermistor sensor in the illustrated embodiment is selectedto have a relatively high anomaly point temperature, and in one example,has an anomaly point temperature of approximately 120C. In addition, asecond thermistor sensor 68 is provided in thermal communication withthe thermistor sensor 10 and functions as a heater. In this connection,the thermistor 6 is directly coupled to the power supply so that it maybe readily heated to a level in excess of its anomaly temperature. inaddition, the thermistor sensor 68 is selected to have a relatively lowanomaly. temperature, in comparison with the thermistor sensor lit), andin one example has an anomaly temperature of approximately 80C incomparison with the 120C anomaly temperature of thermistor sensor 10. Asa result, thermistor sensor 68 is energized to the point whereself-heating occurs substantially in advance of this occurance withrespect to thermistor sensor 110. Consequently, during usual operationof the device when proper coolant flow is present the thermistor sensor68, nevertheless, remains in its self-heating region, and transfers asufficient amount of heat to thermistor sensor Ml so as to maintainthermistor sensor 10 at a relatively high temperature, although belowits anomaly point temperature. Consequently, minor changes in ambienttemperature do not affect the operation of thermistor sensor it), sinceit is maintained at a relatively high temperature, although it retainsits sensitivity to variations in the heat exchange properties of thefluid medium to which it is exposed.

Accordingly, operation of the FIG. 8 embodiment is generally similar tothat of the preceding embodiments. More particularly, when thethermistor sensor Ml ie exposed to a fluid medium having decreased heatexchange properties it senses that change and begins to self-heat anddevelop a substantially increased resistance as its anomaly point isapproached, as previously explained. When the voltage level at junctionM reaches the level V (referring to FIG. .2), the signal is sufficientto activate a suitable switch means '70, such as a silicon controlledrectifier, which includes a gate electrode 71 coupled to the junctionthrough a current limiting resistor 72. in addition, a suitableindicating means 76 is preferably provided connected in theanode-cathode circuit of the silicon controlled rectifier and isactivated when the silicon controlled rectifier is triggered. in thisregard the indicating means may, for example, comprise a suitableindicator light, an alarm buzzer, or the like. Similarly, the indicatingmeans 76 may include a relay coil, for example, which upon energization,effects opening or closing of an associated pair of relay contacts in anexternal load circuit so as to effect a circuit breaking function, whenthe coolant flow rate decreases to a possibly deleterious level.

it may be noted that a system such as that illustrated hereinabove inaccordance with the principles of the present invention is quiteflexible and versatile in use lib and may be utilized in variousconfigurations. For example, the thermistor sensor may be disposed in asuitable gaseous medium, such as air, for sensing a decrease in the flowrate utilized for cooling heat sensitive equipment, since a reduced flowvelocity effects less efficient heat transfer, which in turn disturbsthe stability of the thermistor sensor, as previously explained, andcauses an abrupt rise in the resistance of the thermistor sensor as itis heated. Similarly, the thermistor sensor may be utilized for merelysensing a change in the ambient temperature of the air as an indicationthat the equipment is not being properly cooled. Alternatively, thethermistor sensor may be adapted for disposition in a liquid medium. insuch circumstances the thermistor sensor may be utilized, for example,for sensing a liquid flow rate in the same general manner as it isutilized for sensing a gaseous flow, or, for detecting the level ofliquid in a container. in the latter situation the characteristics ofthe thermistor sensor may be selected such that in the presence of theliquid at a preselected temperature, the thermistor sensorissufficiently cooledflwhile in thermal communication with the liquid,whereas self heating and consequent disturbance of the stability of thethermistor sensor occurs, in the manner previously explained, when thethermistor sensor is not in thermal communication with the liquid.

it may be further noted that in the various embodiments previouslydescribed a transistor and a silicon controlled rectifier have beenindicated as suitable for sensing increased thermistor resistanceincident to a substantial increase in the self-heating of thethermistor, although various other devices may be used in thisconnection, such as a triac, a thyristor, etc. In addition, it should benoted that in those instances, where a transistor resistance sensingswitch is utilized, a nonlatching circuit function is obtained, i.e.,when the turn-on signal is removed from its base electrode thetransistor becomes non-conductive. Similarly, in those instances, wherea silicon controlled rectifier resistance sensing switch is utilized, alatching circuit function results, i.e., once triggered, the siliconcontrolled rectifier remains conductive until the bias potential isremoved from its anode-cathode circuit. Accordingly, it may beappreciated that the selection of the resistance sensing switch elementto be utilized is dependent upon the contemplated circuit function andthe indication or reaction which is desired.

Thus, a novel apparatus has been provided for use in detectingvariations in the heat exchange properties of a fluid medium.

Although several preferred embodiments of the present invention havebeen described in detail it should be understood that variousmodifications and changes will be readily apparent to those skilled inthe art and such changes and modifications are deemed to be within thespirit and scope of the invention as set forth in the appended claims.

What is claimed is:

ll. A device for sensing variations in the heat exchange properties ofafluid medium as an indication of heating mode, calibration means coupledto said thermistor sensor and defining a junction therewith,

energizing means coupled to said thermistor sensor to effect theapplication of sufficient electrical power to said thermistor sensor toenable operation thereof in its self-heating mode so as to establish afirst preselected voltage level at said junction in response to a firstrelative velocity of the medium and to establish a second preselectedvoltage level at said junction in response to a second relative velocityof the medium less than said first relative velocity, said secondpreselected voltage level being associated with operation of saidthermistor sensor in its self-heating mode, 7

voltage responsive means comprising a voltage regulator diode having athreshold voltage substantially equal to said second preselected voltagelevel coupled to said junction for providing an electrical signal onlyin response to the establishment of said second preselected voltagelevel at said junction,

. and i a selectively energizable switch means coupled to said voltageregulator diode and only operable in response to the generation of saidelectrical signal for selectively energizing a load.

2. A device in accordance with claim 1 wherein a plurality of thermistorsensors are provided each being adapted to operate in its self-heatingmode and having a resistance which increases with increasingtemperature, said thermistor sensors being arranged in mutually spacedrelationship with respect to each other at different locations withinthe fluid medium for sensing variations in the heat exchange propertiesassociated with variations inthe relative velocity of the medium at saiddifferent locations.

3. A device in accordance with claim 2 wherein a plurality of steeringdiodes are provided, each of said steering diodes respectivelyconnecting each one of saidthermistor sensors to said voltage regulatordiode for providing selective electrical isolation between saidrespective thermistor sensors and said voltage regulator diode.

4. A device in accordance with claim 1 wherein said thermistor sensorhas a resistance which increases with increasing temperature and saidcalibration means comprises a resistor connected in series relationshipwith said thermistor sensor to define said junction.

5. A device in accordance with claim 4 wherein said switch meanscomprises a silicon controlled rectifier having its gate electrodecoupled to said voltage regulator diode, said silicon controlledrectifier being adapted to be rendered conductive by said voltageregulator diode.

6. A device in accordance with claim 5 wherein an indicating means isprovided coupled to said silicon controlled rectifier, said indicatingmeans being rendered operable in response to conduction of said siliconcontrolled rectifier so as to provide an indication of a variation inthe relative velocity of the medium.

7. A device in accordance with claim 6 wherein means are providedelectrically coupled to said energizing means and thermally coupled tosaid thermistor sensor for heating said thermistor sensor to apreselected temperature below its anomaly point so as to substantiallydesensitize said thermistor sensor from relatively minor changes inambient temperature.

8. A device in accordance with claim 7 wherein said means for heatingsaid thermistor sensor comprises another thermistor sensor adapted tooperate in its selfheating mode and having ananomaly point substantiallybelow that of said thermistor sensor.

9. A device in accordance with claim 4 wherein said switch meanscomprises a selectively energizable relay coil coupled to said siliconcontrolled rectifier and an associated pair of relay contactsoperatively connected to said relay coil, said relay contacts beingadapted to selectively energize a load in response to energization ofsaid relay coil.

10. A device for sensing and indicating variations in the heatexchangeproperties of a fluid medium comprising a thermistor sensor adapted tobe disposed in thermal communication with the medium, said thermistorsensor having a resistance which increases with increasing temperatureand being adapted to operate in a self-heating mode, a resistor coupledto said thermistor sensor to define a voltage divider including ajunction between the resistor and the thermistor sensor, energizingmeans coupled to said thermistor sensor through said resistor to effectthe application of sufficient electrical power to said thermistor sensorto enable operation thereof in its self-heating mode so as to establisha first preselected voltage level at said junction in response to afirst condition of the heat exchange properties of the medium andtoestablish a second preselected voltage level at said junction associatedwith operation of said thermistor in its self-heating mode in responseto a second condition of the heat exchange properties of the medium,said second condition representing a decrease in the heat exchangeproperties of the medium relative to the first condition, first switchmeans including a first transistor coupled to said voltage divider, saidfirst transistor being rendered conductive in response to said firstpreselected voltage level and being rendered non-conductive in responseto said second preselected voltage level, indicating means coupled tosaid first transistor, said indicating means being energized in responseto conduction of said first transistor, voltage responsive means coupledto said junction for providing an electrical signal in response to theestablishment of said second preselected voltage level at said junction,7

second switch means including a second transistor coupled to saidvoltage responsive means, said second transistor being renderedconductive in response to said electrical signal and shunting said firsttransistor so as to render said first transistor non-conductive, and

selectively operable means coupled to said second transistor forproviding an indication in response to conduction of said secondtransistor.

1. A device for sensing variations in the heat exchange properties of afluid medium as an indication of the relative velocity of the mediumcomprising a thermistor sensor adapted to be disposed in thermalcommunication with the medium, said thermistor sensor having aresistance which varies in response to variations in the heat exchangeproperties associated with variations in the relative velocity of themedium and being adapted to operate in a selfheating mode, calibrationmeans coupled to said thermistor sensor and defining a junctiontherewith, energizing means coupled to said thermistor sensor to effectthe application of sufficient electrical power to said thermistor sensorto enable operation thereof in its self-heating mode so as to establisha first preselected voltage level at said junction in response to afirst relative velocity of the medium and to establish a secondpreselected voltage level at said junction in response to a secondrelative velocity of the medium less than said first relative velocity,said second preselected voltage level being associated with operation ofsaid thermistor sensor in its self-heating mode, voltage responsivemeans comprising a voltage regulator diode having a threshold voltagesubstantially equal to said second preselected voltage level coupled tosaid junction for providing an electrical signal only in response to theestablishment of said second preselected voltage level at said junction,and a selectively energizable switch means coupled to said voltageregulator diode and only operable in response to the generation of saidelectrical signal for selectively energizing a load.
 2. A device inaccordance with claim 1 wherein a plurality of thermistor sensors areprovided each being adapted to operate in its self-heating mode andhaving a resistance which increases with increasing temperature, saidthermistor sensors being arranged in mutually spaced relationship withrespect to each other at different locations within the fluid medium forsensing variations in the heat exchange properties associated withvariations in the relative velocity of the medium at said differentlocations.
 3. A device in accordance with claim 2 wherein a plurality ofsteering diodes are provided, each of said steering diodes respectivelyconnecting each one of said thermistor sensors to said voltage regulatordiode for providing selective electrical isolation between saidrespective thermistor sensors and said voltage regulator diode.
 4. Adevice in accordance with claim 1 wherein said thermistor sensor has aresistance which increases with increasing temperature and saidcalibration means comprises a resistor connected in series relationshipwith said thermistor sensoR to define said junction.
 5. A device inaccordance with claim 4 wherein said switch means comprises a siliconcontrolled rectifier having its gate electrode coupled to said voltageregulator diode, said silicon controlled rectifier being adapted to berendered conductive by said voltage regulator diode.
 6. A device inaccordance with claim 5 wherein an indicating means is provided coupledto said silicon controlled rectifier, said indicating means beingrendered operable in response to conduction of said silicon controlledrectifier so as to provide an indication of a variation in the relativevelocity of the medium.
 7. A device in accordance with claim 6 whereinmeans are provided electrically coupled to said energizing means andthermally coupled to said thermistor sensor for heating said thermistorsensor to a preselected temperature below its anomaly point so as tosubstantially desensitize said thermistor sensor from relatively minorchanges in ambient temperature.
 8. A device in accordance with claim 7wherein said means for heating said thermistor sensor comprises anotherthermistor sensor adapted to operate in its self-heating mode and havingan anomaly point substantially below that of said thermistor sensor. 9.A device in accordance with claim 4 wherein said switch means comprisesa selectively energizable relay coil coupled to said silicon controlledrectifier and an associated pair of relay contacts operatively connectedto said relay coil, said relay contacts being adapted to selectivelyenergize a load in response to energization of said relay coil.
 10. Adevice for sensing and indicating variations in the heat exchangeproperties of a fluid medium comprising a thermistor sensor adapted tobe disposed in thermal communication with the medium, said thermistorsensor having a resistance which increases with increasing temperatureand being adapted to operate in a self-heating mode, a resistor coupledto said thermistor sensor to define a voltage divider including ajunction between the resistor and the thermistor sensor, energizingmeans coupled to said thermistor sensor through said resistor to effectthe application of sufficient electrical power to said thermistor sensorto enable operation thereof in its self-heating mode so as to establisha first preselected voltage level at said junction in response to afirst condition of the heat exchange properties of the medium and toestablish a second preselected voltage level at said junction associatedwith operation of said thermistor in its self-heating mode in responseto a second condition of the heat exchange properties of the medium,said second condition representing a decrease in the heat exchangeproperties of the medium relative to the first condition, first switchmeans including a first transistor coupled to said voltage divider, saidfirst transistor being rendered conductive in response to said firstpreselected voltage level and being rendered non-conductive in responseto said second preselected voltage level, indicating means coupled tosaid first transistor, said indicating means being energized in responseto conduction of said first transistor, voltage responsive means coupledto said junction for providing an electrical signal in response to theestablishment of said second preselected voltage level at said junction,second switch means including a second transistor coupled to saidvoltage responsive means, said second transistor being renderedconductive in response to said electrical signal and shunting said firsttransistor so as to render said first transistor non-conductive, andselectively operable means coupled to said second transistor forproviding an indication in response to conduction of said secondtransistor.